1. Field of the Invention
The present invention relates to a semiconductor device that includes an impact ionization control layer that controls the location of impact ionization.
2. Related Art
Semiconductor devices (nitride semiconductor devices) using nitride semiconductors have large dielectric breakdown electric fields, excellent heat resistance, and high electron saturation drift velocity, so that they can provide electronic devices that are excellent in terms of high-temperature or high-power operation, for example, as compared to Si- or GaAs-based devices. For this reason, the development of semiconductor devices using nitride semiconductors is being pursued.
For example, for the manufacture of a semiconductor device (specifically, a field-effect transistor: FET) that is one kind of electronic device, it is necessary to use an electrode that has Schottky characteristics on compound semiconductors (nitride semiconductors) as a gate electrode.
If an electrode having Schottky characteristics is directly joined to a nitride semiconductor, a Schottky leakage current may be generated and may adversely affect the transistor characteristics. Because of this background, the research and development of field-effect transistors having an MIS (metal-insulator semiconductor) structure (hereinafter referred to as “MISFETs”) are being pursued in order to reduce the Schottky leakage current (e.g., JP H10-173203A).
However, in some cases, conventional MISFETs may induce impact ionization (a phenomenon in which electrons accelerated by an electric field collide with the crystal lattice, thereby producing electrons and holes) in a channel that corresponds to the gate electrode.
The holes caused by such impact ionization are hard to absorb with the gate electrode because of the presence of a gate insulating film. Also for the drain electrode and the source electrode, it is difficult to speedily absorb the holes because of their distances from the location of the impact ionization. In other words, the holes are accumulated in the channel that corresponds to the gate electrode, which causes the problem that the current flowing through the channel is affected by the holes, thus inhibiting proper operation.